Part 13

It is a permanent and universal law in vast numbers of unorganised bodies that their composition is definite and invariable, the same compound always consisting of the same elements united together in the same proportions. Two substances may indeed be mixed; but they will not combine to form a third substance different from both, unless their component particles unite in definite proportions; that is to say, one part by weight of one of the substances will unite with one part by weight of the other, or with two parts, or three, or four, &c., so as to form a new substance; but in any other proportions they will only be mechanically mixed. For example, one part by weight of hydrogen gas will combine with eight parts by weight of oxygen gas, and form water; or it will unite with sixteen parts by weight of oxygen, and form a substance called deutoxide of hydrogen; but, added to any other weight of oxygen, it will produce one or both of these compounds mingled with the portion of oxygen or hydrogen in excess. The law of definite proportion established by Dr. Dalton, on the principle that every compound body consists of a combination of the atoms of its constituent parts, is of universal application, and is in fact one of the most important discoveries in physical science, furnishing information previously unhoped for with regard to the most secret and minute operations of nature, in disclosing the relative weights of the ultimate atoms of matter. Thus an atom of oxygen uniting with an atom of hydrogen forms the compound water; but, as every drop of water however small consists of eight parts by weight of oxygen and one part by weight of hydrogen, it follows that an atom of oxygen is eight times heavier than an atom of hydrogen. In the same manner sulphuretted hydrogen gas consists of sixteen parts by weight of sulphur and one of hydrogen; therefore an atom of sulphur is sixteen times heavier than an atom of hydrogen. Also carbonic oxide is constituted of six parts by weight of carbon and eight of oxygen; and, as an atom of oxygen has eight times the weight of an atom of hydrogen, it follows that an atom of carbon is six times heavier than one of hydrogen. Since the same definite proportion holds in the composition of a vast number of substances that have been examined, it has been concluded that there are great differences in the weights of the ultimate particles of matter. Although Dalton’s law is fully established, yet instances have occurred from which it appears that the atomic theory deduced from it is not always maintained. M. Gay Lussac discovered that gases unite together by their bulk or volumes, in such simple and definite proportions as one to one, one to two, one to three, &c. For example, one volume or measure of oxygen unites with two volumes or measures of hydrogen in the formation of water.

Dr. Faraday has proved, by experiments on bodies both in solution and fusion, that chemical affinity is merely a result of the electrical state of the particles of matter. Now it must be observed that the composition of bodies, as well as their decomposition, may be accomplished by means of electricity; and Dr. Faraday has found that this chemical composition and decomposition, by a given current of electricity, is always accomplished according to the laws of definite proportions; and that the quantity of electricity requisite for the decomposition of a substance is exactly the quantity necessary for its composition. Thus the quantity of electricity which can decompose a grain weight of water is exactly equal to the quantity of electricity which unites the elements of that grain of water together, and is equivalent to the quantity of atmospheric electricity which is active in a very powerful flash of lightning. This law is universal, and of that high and general order which characterises all great discoveries. Chemical force is extremely powerful. A pound of the best coal gives when burnt sufficient heat to raise the temperature of 8086 pounds of water one Centigrade degree, whence Professor Helmholtz of Bonn has computed that the magnitude of the chemical force of attraction between the particles of a pound of coal and the quantity of oxygen that corresponds to it, is capable of lifting a weight of 100 pounds to the height of 20 miles.

Dr. Faraday has given a singular instance of cohesive force inducing chemical combination, by the following experiment, which seems to be nearly allied to the discovery made by M. Dœbereiner, in 1823, of the spontaneous combustion of spongy platinum (N. 171) exposed to a stream of hydrogen gas mixed with common air. A plate of platinum with extremely clean surfaces, when plunged into oxygen and hydrogen gas mixed in the proportions which are found in the constitution of water, causes the gases to combine and water to be formed, the platinum to become red-hot, and at last an explosion to take place; the only conditions necessary for this curious experiment being excessive purity in the gases and in the surface of the plate. A sufficiently pure metallic surface can only be obtained by immersing the platinum in very strong hot sulphuric acid and then washing it in distilled water, or by making it the positive pole of a galvanic pile in dilute sulphuric acid. It appears that the force of cohesion, as well as the force of affinity, exerted by particles of matter, extends to all the particles within a very minute distance. Hence the platinum, while drawing the particles of the two gases towards its surface by its great cohesive attraction, brings them so near to one another that they come within the sphere of their mutual affinity, and a chemical combination takes place. Dr. Faraday attributes the effect in part also to a diminution in the elasticity of the gaseous particles on their sides adjacent to the platinum, and to their perfect mixture or association, as well as to the positive action of the metal in condensing them against its surface by its attractive force. The particles when chemically united run off the surface of the metal in the form of water by their gravitation, or pass away as aqueous vapour and make way for others.

The oscillations of the atmosphere, and the changes in its temperature, are measured by variations in the heights of the barometer and thermometer. But the actual length of the liquid columns depends not only upon the force of gravitation, but upon the cohesive force or reciprocal attraction between the molecules of the liquid and those of the tube containing it. This peculiar action of the cohesive force is called capillary attraction or capillarity. If a glass tube of extremely fine bore, such as a small thermometer tube, be plunged into a cup of water or spirit of wine, the liquid will immediately rise in the tube above the level of that in the cup; and the surface of the little column thus suspended will be a hollow hemisphere, whose diameter is the interior diameter of the tube. If the same tube be plunged into a cupful of mercury, the liquid will also rise in the tube, but it will never attain the level of that in the cup, and its surface will be a hemisphere whose diameter is also the diameter of the tube (N. 172). The elevation or depression of the same liquid in different tubes of the same matter is in the inverse ratio of their internal diameters (N. 173), and altogether independent of their thickness; whence it follows that the molecular action is insensible at sensible distances, and that it is only the thinnest possible film of the interior surface of the tubes that exerts a sensible action on the liquid. So much indeed is this the case, that, when tubes of the same bore are completely wetted with water throughout their whole extent, mercury will rise to the same height in all of them, whatever be their thickness or density, because the minute coating of moisture is sufficient to remove the internal column of mercury beyond the sphere of attraction of the tube, and to supply the place of a tube by its own capillary attraction. The forces which produce the capillary phenomena are the reciprocal attraction of the tube and the liquid, and of the liquid particles on one another; and, in order that the capillary column may be in equilibrio, the weight of that part of it which rises above or sinks below the level of the liquid in the cup must balance these forces.

The estimation of the action of the liquid is a difficult part of this problem. La Place, Dr. Young, and other mathematicians, have considered the liquid within the tube to be of uniform density; but M. Poisson, in one of those masterly productions in which he elucidates the most abstruse subjects, has proved that the phenomena of capillary attraction depend upon a rapid decrease in the density of the liquid column throughout an extremely small space at its surface. Every indefinitely thin layer of a liquid is compressed by the liquid above it, and supported by that below. Its degree of condensation depends upon the magnitude of the compressive force; and, as this force decreases rapidly towards the surface, where it vanishes the density of the liquid decreases also. M. Poisson has shown that, when this force is omitted, the capillary surface becomes plane, and that the liquid in the tube will neither rise above nor sink below the level of that in the cup. In estimating the forces, it is also necessary to include the variation in the density of the capillary surface round the edges from the attraction of the tube.

The direction of the resulting force determines the curvature of the surface of the capillary column. In order that a liquid may be in equilibrio, the force resulting from all the forces acting upon it must be perpendicular to the surface. Now it appears that, as glass is more dense than water or alcohol, the resulting force will be inclined towards the interior side of the tube; therefore the surface of the liquid must be more elevated at the sides of the tube than in the centre in order to be perpendicular to it, so that it will be concave as in the thermometer. But, as glass is less dense than mercury, the resulting force will be inclined from the interior side of the tube (N. 174), so that the surface of the capillary column must be more depressed at the sides of the tube than in the centre, in order to be perpendicular to the resulting force, and is consequently convex, as may be perceived in the mercury of the barometer when rising. The absorption of moisture by sponges, sugar, salt, &c., are familiar examples of capillary attraction. Indeed the pores of sugar are so minute, that there seems to be no limit to the ascent of the liquid. Wine is drawn up in a curve on the interior surface of a glass; tea rises above its level on the side of a cup; but, if the glass or cup be too full, the edges attract the liquid downwards, and give it a rounded form. A column of liquid will rise above or sink below its level between two plane parallel surfaces when near to one another, according to the relative densities of the plates and the liquid (N. 175); and the phenomena will be exactly the same as in a cylindrical tube whose diameter is double the distance of the plates from each other. If the two surfaces be very near to one another, and touch each other at one of their upright edges, the liquid will rise highest at the edges that are in contact, and will gradually diminish in height as the surfaces become more separated. The whole outline of the liquid column will have the form of a hyperbola. Indeed, so universal is the action of capillarity, that solids and liquids cannot touch one another without producing a change in the form of the surface of the liquid.

The attractions and repulsions arising from capillarity present many curious phenomena. If two plates of glass or metal, both of which are either dry or wet, be partly immersed in a liquid parallel to one another, the liquid will be raised or depressed close to their surfaces, but will maintain its level through the rest of the space that separates them. At such a distance they neither attract nor repel one another; but the instant they are brought so near as to make the level part of the liquid disappear, and the two curved parts of it meet, the two plates will rush towards each other and remain pressed together (N. 176). If one of the surfaces be wet and the other dry, they will repel one another when so near as to have a curved surface of liquid between them; but, if forced to approach a little nearer, the repulsion will be overcome, and they will attract each other as if they were both wet or both dry. Two balls of pith or wood floating in water, or two balls of tin floating in mercury, attract one another as soon as they are so near that the surface of the liquid is curved between them. Two ships in the ocean may be brought into collision by this principle. But two balls, one of which is wet and the other dry, repel one another as soon as the liquid which separates them is curved at its surface. A bit of tea-leaf is attracted by the edge of the cup if wet, and repelled when dry, provided it be not too far from the edge and the cup moderately full; if too full, the contrary takes place. It is probable that the rise of the sap in vegetables is in some degree owing to capillarity.

SECTION XV.

Analysis of the Atmosphere—Its Pressure—Law of Decrease in Density—Law of Decrease in Temperature—Measurement of Heights by the Barometer—Extent of the Atmosphere—Barometrical Variations—Oscillations—Trade-Winds—Cloud-Ring—Monsoons—Rotation of Winds—Laws of Hurricanes.

THE atmosphere is not homogeneous. It appears from analysis that, of 100 parts, 99·5 consist of nitrogen and oxygen gases mixed in the proportions of 79 to 21 of volume, the remainder consists of 0·05 parts of carbonic acid and on an average 0·45 of aqueous vapour. These proportions are found to be the same at all heights hitherto attained by man. The air is an elastic fluid, resisting pressure in every direction, and is subject to the law of gravitation. As the space in the top of the tube of a barometer is a vacuum, the column of mercury suspended by the pressure of the atmosphere on the surface of that in the cistern is a measure of its weight. Consequently every variation in the density occasions a corresponding rise or fall in the barometrical column. At the level of the sea in latitude 42°, and at the temperature of melting ice, the mean height of the barometer is 29·922 or 30 inches nearly. The pressure of the atmosphere is about fifteen pounds on every square inch; so that the surface of the whole globe sustains a weight of 11,671,000,000 hundreds of millions of pounds. Shell-fish, which have the power of producing a vacuum, adhere to the rocks by a pressure of fifteen pounds upon every square inch of contact.

The atmosphere when in equilibrio is an ellipsoid flattened at the poles from its rotation with the earth. In that state its strata are of uniform density at equal heights above the level of the sea; but since the air is both heavy and elastic, its density necessarily diminishes in ascending above the surface of the earth; for each stratum of air is compressed only by the weight above it. Therefore the upper strata are less dense because they are less compressed than those below them. Whence it is easy to show, supposing the temperature to be constant, that if the heights above the earth be taken in increasing arithmetical progression, that is, if they increase by equal quantities, as by a foot or a mile, the densities of the strata of air, or the heights of the barometer which are proportionate to them, will decrease in geometrical progression. For example, at the level of the sea if the mean height of the barometer be 29·922 inches, at the height of 18,000 feet it will be 14·961 inches, or one half as great; at the height of 36,000 feet it will be one-fourth as great; at 54,000 feet it will be one-eighth, and so on. Sir John Herschel has shown that the actual decrease is much more rapid, and that, in any hypothesis that has been formed with regard to the divisibility of the aërial atoms, a vacuum exists at the height of 80 or 90 miles above the earth’s surface, inconceivably more perfect than any that can be produced in the best air-pumps. Indeed the decrease in density is so rapid that three-fourths of all the air contained in the atmosphere is within four miles of the earth; and, as its superficial extent is 200 millions of square miles, its relative thickness is less than that of a sheet of paper when compared with its breadth. The air even on mountain tops is sufficiently rare to diminish the intensity of sound, to affect respiration, and to occasion a loss of muscular strength. The blood burst from the lips and ears of M. de Humboldt as he ascended the Andes; and he experienced the same difficulty in kindling and maintaining a fire at great heights which Marco Polo, the Venetian, felt on the mountains of Central Asia. M. Gay-Lussac ascended in a balloon to the height of 4·36 miles, and he suffered greatly from the rarity of the air. It is true that at the height of thirty-seven miles the atmosphere is still dense enough to reflect the rays of the sun when 18° below the horizon; but the tails of comets show that extremely attenuated matter is capable of reflecting light. And although, at the height of fifty miles, the bursting of the meteor of 1783 was heard on earth like the report of a cannon, it only proves the immensity of the explosion of a mass half a mile in diameter, which could produce a sound capable of penetrating air three thousand times more rare than that we breathe. But even these heights are extremely small when compared with the radius of the earth.

The density of the air is modified by various circumstances, chiefly by changes of temperature, because heat dilates the air and cold contracts it, varying 1/480 of the whole bulk when at 32° for every degree of Fahrenheit’s thermometer. Experience shows that the heat of the air decreases as the height above the surface of the earth increases. It appears that the mean temperature of space is 226° below the zero point of Fahrenheit by the theories of Fourier and Pouillet, but Sir John Herschel has computed it to be -239° Fahr. from observations made during the ascent in balloons. Such would probably be the temperature of the surface of the earth also, were it not for the non-conducting power of the air, whence it is enabled to retain the heat of the sun’s rays, which the earth imbibes and radiates in all directions. The decrease in heat is very irregular; each authority gives a different estimate, because it varies with latitude and local circumstances, but from the mean of five different statements it seems to be about one degree for every 334 feet; the mean of observations made in balloons is 400 feet, which is probably nearer the truth. This is the cause of the severe cold and perpetual snow on the summits of the alpine chains. In the year 1852 four ascents in a balloon took place from the meteorological observatory at Kew, in which the greatest height attained was 22,370 feet. The observations then made by Mr. Welsh furnished Sir John Herschel with data for computing that the temperature of space is minus 239°, that is 239° below the zero point of Fahrenheit, that the limiting temperature of the atmosphere is probably 77-1/2 degrees below that point at the equator, and 119-1/2 below it at the poles, with a range of temperature from the surface of 161-1/2° in the former case, and 119-1/2° in the latter. During these ascents it was found that the temperature of the air decreases uniformly up to a certain point, where it is arrested and remains constant, or increases through a depth of 2000 or 3000 feet, after which it decreases again according to the same law as before. Throughout this zone of constant temperature it either rains, or there is a great fall in the dew point; in short, it is the region of clouds, and the increase of temperature is owing to the latent or absorbed heat set free by the condensation of the aqueous vapour. In the latitude of Kew the cloud region begins at altitudes varying between 2000 and 6500 feet, according to the state of the weather.

Were it not for the effects of temperature on the density of the air, the heights of mountains might be determined by the barometer alone; but as the thermometer must also be consulted, the determination becomes more complicated. Mr. Ivory’s method of computing heights from barometrical measurements has the advantage of combining accuracy with the greatest simplicity. Indeed the accuracy with which the heights of mountains can be obtained by this method is very remarkable. Admiral Smyth, R.N., and Sir John Herschel measured the height of Etna by the barometer, without any communication and in different years; Admiral Smyth made it 10,874 feet, and Sir John Herschel 10,873, the difference being only one foot. In consequence of the diminished pressure of the atmosphere water boils at a lower temperature on mountain tops than in the valleys, which induced Fahrenheit to propose this mode of observation as a method of ascertaining their heights. It is very simple, as Professor Forbes ascertained that the temperature of the boiling point varies in arithmetical proportion with the height, or 5495 feet for every degree of Fahrenheit, so that the calculation of height becomes one of arithmetic only, without the use of any table.

The mean pressure of the atmosphere is not the same all over the globe. It is less by 0·24 of an inch at the equator than at the tropics or in the higher latitudes, in consequence of the ascent of heated air and vapour from the surface of the ocean. It is less also on the shores of the Baltic Sea than it is in France, and it was observed by Sir James C. Ross that throughout the whole of the Antarctic Ocean, from 68° to 74° S. latitude, and from 8° to 7° W. longitude, there is a depression of the barometer amounting to an inch and upwards, which is equivalent to an elevation above the sea level of 800 feet. A similar depression was observed by M. Erman in the sea of Ochotzk, and in the adjacent continent of eastern Siberia. Sir John Herschel assigns as the cause of these singular anomalies the great system of circulation of the trade and antetrade winds, in both hemispheres, reacting upon the general mass of the continents as obstacles in their path, which is modified by the configuration of the land.

There are various periodic oscillations in the atmosphere, which, rising and falling like waves in the sea, occasion corresponding changes in the height of the barometer, but they differ as much from the trade-winds, monsoons, and other currents, as the tides of the sea do from the Gulf-stream and other oceanic rivers. The sun and moon disturb the equilibrium of the atmosphere by their attraction, and produce annual undulations which have their maximum altitudes at the equinoxes, and their minima at the solstices. There are also lunar tides, which ebb and flow twice in the course of a lunation. The diurnal tides, which accomplish their rise and fall in six hours, are greatly modified by the heat of the sun. Between the tropics the barometer attains its maximum height about nine in the morning, then sinks till three or four in the afternoon; it again rises and attains a second maximum about nine in the evening, and then it begins to fall, and reaches a second minimum at three in the morning, again to pursue the same course. According to M. Bouvard, the amount of the oscillations at the equator is proportional to the temperature, and in other parallels it varies as the temperature and the square of the cosine of the latitude conjointly; consequently it decreases from the equator to the poles, but it is somewhat greater in the day than in the night.